[JaTochNietDan]

You need to make a variable to track if the player has a made a vehicle or not already, then store the ID of the vehicle in that variable which you can later delete or do whatever you want with it, for example:

pawn Код:

new iSpawnedCar[MAX_PLAYERS] = -1;

pawn Код:

if(strcmp("/race", cmdtext, true) == 0)
{
    if(iSpawnedCar[playerid] != -1) DestroyVehicle(iSpawnedCar[playerid]);
    SetPlayerInterior(playerid,7);
    SendClientMessage(playerid, 0x00FFFFAA, "You've been teleported to Race.");
    SetPlayerCheckpoint(playerid, -1394.54,-243.56,1043.20, 13.0);
    iSpawnedCar[playerid] = CreateVehicle(415,-1394.54,-243.56,1043.20, 180.0, -1, -1, -1);
    LinkVehicleToInterior(iSpawnedCar[playerid],7);
    PutPlayerInVehicle(playerid,iSpawnedCar[playerid], 0);
    return 1;
}

Additionally I removed the completely un-needed additional strcmp check, not sure why you added that!